1. Field of the Invention
The present invention relates to a polymeric fluorescent substance for organic electroluminescence devices (which may hereinafter be referred to as EL devices) and the EL devices produced by using such a substance. More particularly, the invention pertains to a polymeric fluorescent substance which is soluble in solvents and has strong fluorescence and the organic EL devices with high luminous efficiency produced by using said fluorescent substance.
2. Description of the Related Art
The inorganic electroluminescence devices (which may hereinafter be referred to as inorganic EL devices) using an inorganic fluorescent substance as light emitting material have been applied to various uses, for example, as flat light source for back-light of liquid crystal displays and various display devices such as flat panel displays, but in these devices a high-voltage alternating current has been required for driving the devices.
Recently, Tang et al manufactured an organic EL device having a double-layer structure comprising a laminate of a light emitting layer made of an organic fluorescent dye and a layer of an organic charge transport compound generally used in a photosensitive layer for electrophotography to realize a low-voltage-drive, high-efficiency and high-luminance organic EL device. In view of many advantageous features of organic EL devices as compared with inorganic EL devices, such as low-voltage drive, high luminance and easy luminescence of a large number of colors, various attempts have been made and reported regarding development and improvements of structure of the devices, organic fluorescent dyes, organic charge transport compounds, etc. (Jpn. J. Appl. Phys., Vol. 27, L269, 1988; J. Appl. Phys., Vol. 65, 3610, 1989, etc.).
Hitherto, low-molecular weight organic fluorescent dyes have been generally used as material of the light emitting layer, and regarding polymeric light emitting materials, proposals have been made in some patents and publications such as WO9013148, JP-A-3-244630 and Appl. Phys. Lett., Vol. 58, p. 1982 (1991). WO9013148 discloses an EL device using a thin film of poly(p-phenylene vinylene) obtained by forming a film of a soluble precursor on the electrode and subjecting it to a heat treatment to convert the precursor into a conjugated polymer.
JP-A-3-244630 illustrates conjugated polymers having a salient feature that they are themselves soluble in solvents and unnecessitate heat treatment. In Appl. Phys. Lett., Vol. 58, p. 1982 (1991), are also disclosed polymeric light emitting materials soluble in solvents and organic EL devices made by using such materials. Most of the organic EL devices manufactured by using these materials, however, were not satisfactorily high in luminous efficiency.
Regarding quantum yield of fluorescence of poly(p-phenylene vinylene) (which may hereinafter be referred to as PPV), it has been reported that the quantum yield of fluorescence and EL intensity can be improved by using a copolymer of PPV in which a non-conjugated segment is interposed in the conjugated segment. Control of the chain length of the conjugated segment and non-conjugated segment is made by making use of a difference in decomposition temperature of the precursors of the conjugated polymers used for the respective segments (Nature, Vol. 356, p. 47 (1992)). It is also disclosed that by heat treating a copolymer of p-phenylene-ethylene-sulfonium salt and 2,5-dimethoxy-p-phenylene-methoxy-ethylene, there is produced a poly(p-phenylene vinylene) portion encompassed by the 2,5-dimethoxy-p-phenylene-methoxy-ethylene-methoxy-ethylene moiety, and this elevates EL intensity as compared with PPV homopolymer. Here, since the 5-dimethoxy-p-phenylene-methoxy-ethylene moiety in the copolymer is hard to decompose by heat treatment alone, it is utilized for inhibiting the copolymer from being converted in its entirety into a conjugated polymer by the heat treatment.
It is further reported that a polymer in which the conjugated monomeric moiety with strong fluorescence and aliphatic hydrocarbons are coupled by ether linkage generates blue fluorescent light because of its short conjugated chain length, and a blue light-emitting organic EL device can be made by using said polymer (Macromolecules, Vol. 26, p. 1188, 1993).
However, in manufacture of the organic EL devices using polymers which have hitherto been reported, it was necessary to mold a soluble precursor into a thin film and convert its structure into a conjugated polymeric structure by high-temperature heat treatment, so that there have been restrictions on the material usable for the associated parts such as substrate in manufacture of the device. Also, when a precursor polymer is converted into a conjugated polymer by heat treatment, control of the non-conjugated moiety in the polymer has been made by adjusting the heat treatment conditions, so that such control has not been perfect, and when said polymer was used for an organic EL device, it might suffer a structural change in long-time continuous run.
When using a polymer in which the conjugated monomeric moiety is coupled with non-conjugated aliphatic hydrocarbons by ether linkage, the obtained thin film is high in resistance and low in current density, so that high luminance can hardly be obtained in the EL devices using the said polymer.
In the case of soluble conjugated polymers, although no high-temperature heat treatment is necessary after formation of the thin film, further improvements are required on luminance and luminous efficiency of the produced EL devices.
Thus, an organic EL device which can be easily made according to a coating method by using a polymeric fluorescent substance having good solubility in solvents, high quantum yield of fluorescence and excellent electric conductive property has been desired.